In a ring link, the various stations S1 to Sn (FIG. 1) are interconnected in series by a ring A. A message transmitted by any one station transits through each of the other stations connected to the ring prior to returning to the transmitting station. The destination station for the message copies it as it goes past and retransmits the message over the ring plus various bits including a "message-copied" flag.
Each station may be inserted on the ring or withdrawn therefrom. When a station is withdrawn, as shown diagrammatically in FIG. 2, ring continuity is ensured by an insertion relay RI carried by the coupler card 1 of the station, said coupler card being provided with a transmission circuit 2 and a reception circuit 3. The coupler card is connected to the ring A by means of connectors 4 and 5, with the ring being constituted between stations (FIG. 1) by a cord provided with a connector at each end. At rest, the insertion relay serves firstly to ensure metallic continuity of the ring and secondly to loop the transmission circuit 2 back onto the reception circuit 3.
This architecture suffers from a major drawback: when a coupler card or a cord is withdrawn, the ring opens and ceases to operate until the situation is restored, i.e. until the coupler card or the cord is put back into place. This cannot be tolerated when the maximum network down-time in a network using this type of coupler is required to be less than one second.
The object of the invention is to mitigate this defect by limiting the down-time of the ring to about 100 milliseconds.